Preheat the oven to 300°F (150°C).
Sprinkle the ground hazelnuts evenly over the bottom of the tart crust.
Peel and core the apples, then cut each into 6–8 wedges, depending on their size. Toss with half the sugar (75 g) and the cinnamon, and let stand for about 15 minutes.
Bake for approximately 15 minutes, or until the apples begin to soften. Remove from the oven and allow to cool slightly.
Meanwhile, increase the oven temperature to 350°F (180°C).
In a bowl, beat the egg and remaining sugar (75 g) until pale and foamy. Add the flour and whisk until smooth and free of lumps. Stir in the heavy cream, followed by the Calvados, if using.
Pour the custard mixture over the apples, filling the tart until the custard reaches approximately two-thirds of the way up the apple wedges.
Bake for 15 minutes, then sprinkle a little extra sugar over the top. Return to the oven for another 15 minutes, or until the apples are tender, the custard is set, and the top is golden brown and lightly caramelized.
Carefully remove the tart from the pan and transfer it to a wire rack to cool. Serve slightly warm, ideally with vanilla sauce or whipped cream.